When Plants Become Refugees

Getting out of harm’s way isn’t easy when you’re a plant. If the water is rising or a fire is approaching, anything that can run, fly or slither can at least move to higher ground. But trees and other vegetation are pretty much stuck. That’s at least true with high-speed, real-time dangers like floods, but a slow motion disaster—global warming, say—is a different matter. And a new study from the University of Basel in Switzerland is showing how plant species, in their own creeping way, are fleeing rising temperatures and taking up residence where things are cooler.

Organisms that live in a fixed spot are very vulnerable to what’s known as microhabitats. A patch of ground that’s partly shaded by a stand of trees may be deadly for a plant species that needs direct sun, but perfect for one that likes things darker. Just a couple of feet away, however, the survival equation is reversed. The more varied the terrain is, the more microhabitats there will be, which is why an open field may be home to a narrower range of plant species than a patch of rolling woodlands. For pure variety, however, nothing matches a mountain.

The rugged terrain of a mountain face not only includes all manner of crevices, overhangs and exposed slopes, but is also a place of steadily graduated temperatures. The higher you go, the cooler things get and the more you escape heat and humidity pooling below. To test how plants that root near mountains taken advantage of this thermal variety, Basel researchers Daniel Scherrer and Christian Körner set out high-resolution cameras and infrared sensors across patches of Alpine terrain over a two year seasonal cycle. The surveyed area was at an elevation of about 2,500 meters (8,200 ft.) — high enough to get a break from excessive heat below but low enough to be plenty comfortable for plants.

Scherrer and Körner knew in advance the precise temperatures and light conditions various Alpine plant species preferred and over the course of their study sought to determine if spot by spot, the proper plants populated the proper patches of ground. As they expected, that’s just what happened. “We found that the occurrences of plant species across these mosaics of warmth match with their known temperature preferences,” Körner said.

The idea that plants take root in spots particularly suited to them may not be surprising, but what was news — and good news, in fact — was that a number of the species tracked in the study were not necessarily native to those elevations. Instead, they have been emigrating up the slopes as the world slowly warms, finding safe spots in an unstable time.

Said Körner: “This means that rugged Alpine terrain offers refuge habitats — or at least stepping stones to these — at short distance for both small plants and animals.”

For the near term future, this should help most plant species maintain microhabitats in a warmer world. Indeed, Körner and Scherrer ran a computer simulation in which they posited a planet in which mean temperatures had risen 2°C (3.6°F) and found that only about 3% of all thermal microhabitats would vanish. In some cases, the increased warmth would actually lead to greater species diversity.

But the near future is not the more distant future and that’s where the real peril lies. Plants can climb uphill only so long. At some point you run out of mountain — and at that point, you start running out of plants.